Nervous disorders affect millions of people, causing death and a degradation of life. Nervous system disorders include disorders of the central nervous system, peripheral nervous system, and mental health and psychiatric disorders. Such disorders include, for example, without limitation, epilepsy, Parkinson's disease, essential tremor, dystonia, and multiple sclerosis (MS).
Additionally, nervous system disorders include mental health disorders and psychiatric disorders which also affect millions of individuals and include, but are not limited to, anxiety (such as general anxiety disorder, panic disorder, phobias, post traumatic stress disorder (PTSD), and obsessive compulsive disorder (OCD), mood disorders (such as major depression, bipolar depression, and dysthymic disorder), sleep disorders (narcolepsy), eating disorders such as obesity, and anorexia. As an example, epilepsy is the most prevalent serious neurological disease across all ages. Epilepsy is a group of neurological conditions in which a person has or is predisposed to recurrent seizures. A seizure is a clinical manifestation resulting from excessive hypersynchronous, abnormal electrical or neuronal activity in the brain. A neurological event is an activity that is indicative of a nervous system disorder. A seizure is a type of neurological event. This electrical excitability of the brain may be likened to an intermittent electrical overload that manifests with sudden, recurrent and transient changes of mental function, sensations, perceptions, or involuntary body movement. Because the seizures are unpredictable, epilepsy affects a person's employability, psychosocial life, and ability to operate vehicles or power equipment. It is a disorder that occurs in all age groups, socioeconomic classes, cultures, and countries.
Nervous system disorders are often associated with co-morbid life-threatening conditions, which in some cases may have a cardiac cause. For example, a subset of epilepsy patients are at risk for SUDEP. SUDEP is defined as sudden, unexpected, often unwitnessed, non-traumatic and non-drowning death in patients for which no cause has been found except for the individual having a history of seizures. Depending on the cohort studied, SUDEP is responsible for 2% to 18% of all deaths in patients with epilepsy, and the incidence may be up to 40 times higher in young adults with epilepsy than among persons without seizures.
Although the pathophysiological mechanisms leading to death are not fully understood, experimental, autopsy and clinical evidence implicate seizure related heart and pulmonary dysfunction or indicators. Pulmonary events may include obstructive sleep apnea (OSA), central apnea, and neurogenic pulmonary edema. Cardiac events may include cardiac arrhythmic abnormalities including sinus arrhythmia, sinus pause, premature atrial contraction (PAC), premature ventricular contraction (PVC), irregular rhythm (wandering pacemaker, multifocal atrial tachycardia, atrial fibrillation), asystole or paroxysmal tachycardia. Cardiac events may also include conduction abnormalities including AV-block (AVB) and bundle branch block (BBB) and repolarization abnormalities including T-wave inversion and ST-elevation or depression. Lastly, hypertension, hypotension and vaso-vagal syncope (VVS) are common in epilepsy patients.
Epileptic seizures are associated with autonomic neuronal dysfunction that result in a broad array of abnormalities of cardiac and pulmonary function. Different pathopsysiological events may contribute to SUDEP in different patients, and the mechanism is probably multifactorial. Without intervention, respiratory events, including airway obstruction, central apnea and neurogenic pulmonary edema are probably terminal events. In addition, cardiac arrhythmia and anomalies, during the ictal and interictal periods, leading to arrest and acute cardiac failure also plays an important role in potentially terminal events. For example, the paper “Electrocardiographic Changes at Seizure Onset”, Leutmezer, et al., Epilepsia 44(3): 348-354, 203 describes cardiovascular anomalies, such as heart rate variability (HRV), tachycardia and bradycardia, that may precede, occur simultaneous or lag behind EEG seizure onset. “Cardiac Asystole in Epilepsy: Clinical and Neurophysiologic Features”, Rocamora, et al., Epilepsia 44(2): 179-185, 2003 reports that cardiac asystole is “provoked” by the seizure. “Electrocardiograph QT Lengthening Associated with Epileptiform EEG Discharges—a Role in Sudden Unexplained Death in Epilepsy”, Tavemor, et al., Seizure 5(1): 79-83, March 1996 reports QT lengthening during seizures in SUDEP patients versus control. “Effects of Seizures on Autonomic and Cardiovascular Function”, Devinsky Epilepsy Currents 4(2): 43-46, March/April 2004 describes ST segment depression and T-wave inversion, AVB, VPC and BBB during or immediately after a seizure. “Sudden Unexplained Death in Children with Epilepsy”, Donner, et al., Neurology 57: 430-434, 2001 reports that bradycardia is frequently preceded by hypoventilation or apnea suggesting that heart rate changes during seizures may be a result of cardiorespiratory reflexes. Lastly, “EEG and ECG in Sudden Unexplained Death in Epilepsy”, Nei, et al., Epilepsia 45(4) 338-345, 2004 reports on sinus tachycardia during or after seizures.
Patients with a psychiatric illness, such as major depressive disorder (MMD), are also at higher risk for developing cardiac problems. A strong link between depression and cardiac death has been established. Some have postulated cardiac vagal control (CVC) is impaired in patients with depression. Others have shown major depression is associated with reduced baroreflex sensitivity (BRS), a predisposing factor for sudden cardiac death. Anxiety, which is central to OCD, has also been shown to be related with a reduction in baroreflex receptor activity. Thus, evidence suggests autonomic function is altered in depression patients, with decreased vagal activity and/or increased sympathetic arousal as possible explanations for the observed cardiac problems.
Treatment therapies for epilepsy, psychiatric illness, and other nervous system disorders can include any number of possible modalities alone or in combination including, for example, electrical stimulation, magnetic stimulation, drug infusion, and/or brain temperature control. Each of these treatment modalities can be operated using an open loop scheme, where therapy is continuously or intermittently delivered based on preprogrammed schedule. Alternatively, a closed-loop scheme may be used, in which the therapy is delivered to the patient based on information coming from a sensed signal.
An exemplary closed-loop feedback control technique includes receiving from a monitoring element a neurological signal that carries information about a symptom, a condition, or a nervous system disorder. The neurological signal can include, for example, electrical signals (such as EEG, EcoG, and/or EKG), chemical signals, other biological signals (such as change in quantity or neurotransmitters), temperature signals, pressure signals (such as blood pressure, intracranial pressure or cardiac pressure), respiration signals, heart rate signals, pH-level signals, and peripheral nerve signals (cuff electrodes on a peripheral nerve). Monitoring elements include, for example, recording electrodes or various types of sensors.
Standard diagnostic EEG sensing requires two electrodes in contact with body tissue. The first electrode is placed near the desired source of the electrical activity that the physician desires to monitor, and is referred to as active. The second electrode, referred to as the reference, is typically placed outside of the cranium away from the desired source of electrical activity. For example, the reference electrode may be attached to the ear or mastoid, or at the back of the head. Such locations are considered “inactive” since sensing from these areas produces a potential that is close to zero. In a monitoring system or device, differential amplifiers measure the voltage difference between the reference electrode and other active electrodes located within the brain. The resulting intracranial signals are amplified and displayed as channels of EEG activity.
For implantable devices that perform EEG sensing, it is desirable to have the reference electrode contained within the body. If the signals are to be used for seizure detection, it is desirable that the reference electrode be remote from the seizure focus. The active electrodes are positioned either in direct or indirect contact with brain structures affecting a neurological condition for which sensing is being performed. For example, to treat epilepsy the active electrodes may be implanted in brain tissue at or near the seizure focus where they can sense EEG signals, detect a seizure event, and provide stimulation therapy. Conversely, the active electrodes may be positioned in an anatomical target distant from the seizure focus, but which is connected to the seizure focus by way of neuronal pathway projections. Activating pathway projections with electrical stimulation from a distant site (i.e., thalamus) may influence seizure activity at the focus (i.e., hippocampus/amygdala). Similarly, to treat a psychiatric illness such as major depressive disorder or OCD, the active electrodes may be implanted directly in brain tissue involved in mood regulation for depression or OCD, such as the internal capsule or subgenual cingulate cortex (Area 25). Alternatively, the active electrodes may be positioned at distant sites within the limbic-cortical circuit, which have connections to these mood regulating regions of the brain. With either approach, it is desirable to have a single electrode positioned away from the active electrodes, which can function as a reference for EEG sensing and/or function as an indifferent electrode for monopolar stimulation.
For patients being treated for a neurological condition, it is desirable to further refine the monitoring and treatment of neurological events, and when appropriate, provide treatment for co-morbid cardiac detected events. To achieve this, what is desired is a neurostimulation device that treats neurological conditions while monitoring the patient's cardiac state, with provisions to enable cardiac therapy if deemed necessary.
In addition, for some patients it would be beneficial to be able to regulate the cardiac therapy using brain state information, particularly in cases where the change in cardiac state is part of the neurological event. For example, some changes in heart rate function during a neurological event, such as a seizure or panic attack (i.e., tachy episodes, brief asystolic pauses, etc.) are considered normal—the body's reaction to dealing with the event—and do not necessitate cardiac treatment. Inadvertent pacemaker activation during such episodes may interfere with physiological processes aimed at terminating them. Thus, means to regulate the cardiac therapy during seizures and other neurological states would be beneficial.
Currently, there are no neurological devices that perform these functions.